1. Field of the Invention
The present invention relates to an image forming apparatus having a mechanism for detecting a mark on a belt to prevent the misregistration of a toner image to be formed.
2. Description of the Related Art
In recent years, a tandem image forming apparatus has been proposed as a full-color image forming apparatus using an electrophotographic process. The tandem image forming apparatus includes a plurality of photosensitive drums arranged in a row, which respectively correspond to toner colors, to sequentially superimpose toner images on an image bearing belt (intermediate transfer belt), thereby obtaining a desired image.
FIG. 9 illustrates a schematic configuration of an image forming apparatus in the background art. The image forming apparatus illustrated in FIG. 9 is provided with image forming units (1Y, 1M, 1C and 1K) for yellow (Y), magenta (M), cyan (C) and black (K) toners, respectively. In the respective image forming units 1, toner images are sequentially transferred onto an intermediate transfer belt 18.
The transfer of the toner images onto the intermediate transfer belt 18 is performed in nip portions (primary transfer portions N) between photosensitive drums (2Y, 2M, 2C and 2K) respectively provided for the image forming units 1 and primary transfer rollers (5Y, 5M, 5C and 5K).
Specifically, a transfer bias is applied from a primary transfer bias supply (not illustrated) to each of the primary transfer rollers 5 to transfer the toner image from a surface of the photosensitive drum 2 onto the intermediate transfer belt 18 by an electrostatic force. The toner images on the surfaces of the photosensitive drums 2 are formed by the following process.
The surfaces of the photosensitive drums 2 are uniformly charged by charging rollers (3Y, 3M, 3C and 3K) provided in contact with the photosensitive drums 2. A modulated laser beam is emitted from each of exposure devices (7Y, 7M, 7C and 7K) based on image information to form an electrostatic latent image on the surface of each of the photosensitive drums 2.
For the electrostatic latent images respectively formed on the surfaces of the photosensitive drums 2, development units (4Y, 4M, 4C and 4K) respectively containing the yellow (Y), magenta (M), cyan (C) and black (K) toners feed the toners to visualize the electrostatic latent images as the toner images. Then, in each of the primary transfer portions N, the toner image is transferred onto the intermediate transfer belt 18. The residual toner remaining on the surfaces of the photosensitive drums 2 without being transferred to the intermediate transfer belt 18 is cleaned by each of cleaning units (6Y, 6M, 6C and 6K).
The toner image transferred onto the intermediate transfer belt 18 is conveyed to a secondary transfer portion M (nip portion between a secondary transfer opposing roller 12 and a secondary transfer roller 14) with the movement of the intermediate transfer belt 18 to be transferred onto a sheet material. The intermediate transfer belt 18 is movably looped around a driving roller 11, the secondary transfer opposing roller 12, and a tension roller 13. In other words, a driving roller 11 and a tension roller 13 are stretching members for stretching the intermediate transfer belt 18. The driving roller 11 is rotationally driven to move the intermediate transfer belt 18 in a direction indicated by an arrow of FIG. 9.
For transferring the toner image onto the sheet material in the secondary transfer portion M, the sheet materials fed one by one from a feeding portion (not illustrated) are temporarily stopped to wait between a registration roller pair 20 provided before the secondary transfer portion M.
Thereafter, the registration roller pair 20 is rotated according to arrival timing of the toner image at the secondary transfer portion M to feed the sheet material to the secondary transfer portion M, thereby transferring the toner image to a desired position of the sheet material.
If a deviation occurs between the timing for feeding the sheet material to the secondary transfer portion M and the timing for conveying the toner image on the intermediate transfer belt 18 to the secondary transfer portion M, it becomes difficult to transfer the toner image to a desired position of the sheet material. As a result, defect such as image quality degradation occurs.
In the image forming apparatus in this background art, on the assumption that a circumferential length of the intermediate transfer belt and a conveying speed of the toner image remain constant, a time length required for the toner image to reach the secondary transfer portion M is calculated based on writing start timing of the electrostatic latent image onto the photosensitive drum.
Specifically, a period of time required for the toner image to reach the secondary transfer portion M is pre-calculated. The registration roller pair feeds the sheet material to the secondary transfer portion M based on the calculated time to synchronize the arrival timing of the toner image with the conveying timing of the sheet material.
In this case, on the assumption that the circumferential length of the intermediate transfer belt remains constant, the time required for the toner image to reach the secondary transfer portion M is calculated. Therefore, it is difficult to cope with the occurrence of expansion and contraction of the intermediate transfer belt due to an environmental change such as a change in temperature or humidity or a change with time.
Specifically, when the circumferential length of the intermediate transfer belt changes, the time required for the toner image on the intermediate transfer belt to reach the secondary transfer portion also changes. Therefore, the deviation between the arrival timing of the toner image and the conveying timing of the sheet material is occurred, and the image quality degrades.
Therefore, as described in Japanese Patent Application Laid-Open No. 2001-215857, the invention relating to an image forming apparatus for correcting a deviation between the arrival timing of the toner image and the conveying timing of the sheet material to cope with a variation in circumferential length of the intermediate transfer belt is presented.
Japanese Patent Application Laid-Open No. 2001-215857 discloses a constitution including a mark provided at one position on the intermediate transfer belt and a sensor for detecting the passage of the mark. The passage of the mark is detected with the sensor to calculate a circumferential length variation of the intermediate transfer belt.
However, the following problem arises in the image forming apparatus described in Japanese Patent Application Laid-Open No. 2001-215857.
When the passage of the mark provided at one position on the intermediate transfer belt is detected to calculate the circumferential length variation of the intermediate transfer belt, the intermediate transfer belt moves approximately two circumferential lengths to measure the circumferential length of the intermediate transfer belt at some detection start timing.